The present invention relates to a thin-film capacitor device and a RAM device which use a ferroelectric film consisting of, e.g., a ferroelectric material having a perovskite crystal structure.
Recently, memory devices (ferroelectric memories) using a thin ferroelectric film as a storage medium have been developed, and some have already been used in practical applications. The ferroelectric memory is nonvolatile and does not lose its memory contents after the power supply is turned off. When the film is sufficiently thin, spontaneous polarization can be quickly inverted to enable high-speed writes and reads, like the DRAM. Further, a large capacity is also attained because a 1-bit memory cell can be constituted by one transistor and one ferroelectric capacitor.
A thin ferroelectric film suitable for the ferroelectric memory is required to have large remanent polarization, low temperature dependency of remanent polarization, a long holding time (retention) of remanent polarization, and the like.
At present, as a ferroelectric material, lead zirconate titanate (PZT) is mainly used. PZT is a solid solution of lead zirconate and lead titanate. PZT having a molar ratio of almost 1:1 is considered to be excellent as a storage medium because its spontaneous polarization is large to allow reversal even with a low electric field. Since the transition temperature (Curie temperature) of PZT between the ferroelectric phase and the paraelectric phase is as relatively high as 300.degree. C. or higher, the polarization are hardly lost by heat within the use temperature range (120.degree. C. or lower) of a normal electronic circuit.
However, a high-quality thin PZT film is difficult to fabricate due to the following reason. First, since lead as the main component of PZT easily evaporates at 500.degree. C. or higher, the composition is difficult to accurately control in sputtering and subsequent annealing. Second, PZT exhibits ferroelectricity only when it forms a perovskite crystal structure. PZT having this perovskite crystal structure is rarely obtained, and a crystal structure called pyrochlore is easily obtained. When PZT is applied to a silicon device, lead as the main component is difficult to be prevented from diffusing into silicon, and is easily reduced in a reduction atmosphere in the device process to lose the ferroelectricity.
Barium titanate (BaTiO.sub.3) is known as a typical ferroelectric, other than PZT. Barium titanate also has a perovskite crystal, like PZT, and its Curie temperature is about 120.degree. C., as is known well. Since Ba rarely evaporates as compared to Pb, the composition is relatively easily controlled in forming a thin barium titanate film. When barium titanate crystallizes, most crystal structures are of the perovskite type.
Regardless of these advantages, a barium titanate thin-film capacitor is hardly examined as the storage medium of the ferroelectric memory because the remanent polarization of barium titanate is smaller than that of PZT, and the temperature dependency of its remanent polarization is high. This is because the Curie temperature of barium titanate is low (120.degree. C.). When a ferroelectric memory is formed using barium titanate, its memory contents may be lost during use at a high temperature of 100.degree. C. or higher. In addition, the temperature dependency of remanent polarization is high even in the normal use temperature range (85.degree. C. or lower) of an electronic circuit, and the operation is unstable. Therefore, a thin-film capacitor using a thin ferroelectric film of barium titanate is considered to be unsuitable as the storage medium of the ferroelectric memory.